Chemistry of High Temperature Superconductors

The following sections are included:

• PREFACE

• CONTENTS

This article highlights the importance of crystal chemistry in controlling the superconductive properties of the copper oxides. Stabilization of their intergrowth structures requires bond-length matching across the intergrowth interface. At least four different structures -- T/O, T', T* and T ” -- are stabilized in the simplest system La_2−yLn_yCuO₄ (Ln = lanthanide) depending upon the size of the Ln atom and the value of y. Whether the sheets of a parent intergrowth structure can be oxidized or reduced to give p-type or n-type superconductors is determined by the basal plane Cu-O distances as well as the oxygen coordination number around the Cu atoms. The internal electric field created by the formal charges in the adjacent layers as well as the preferred oxidation states for different coordination geometries around Cu appear to modulate the distribution of holes between active and inactive layers, e.g. in YBa₂Cu₃O_6+x. Realization of superconductivity requires the maintenance of a uniform periodic potential -- i.e. a uniform oxygen coordination -- around the Cu atoms. Although the overlap of a localized 4fⁿ level by the conduction band suppresses superconductivity, as in Y_1−zPr_zBa₂Cu₃O_6+x, overlap of a Tl-6s band does not. A thorough investigation of the chemistry-structure-property relationships for the simplest system La_2−ySr_yCuO₄ has led to the identification of the charge fluctuations that separate antiferromagnetic-spin fluctuations from superconductive regions as a result of strong electron-phonon interactions in these materials.

The highly closely related issues of oxygen nonstoichiometry, extended defects and valence state of the metallic elements are essential for superconductivity in the layered copper oxides. Review and classification of the main nonstoichiometry features are presented here. A particular attention is paid to the oxygen nonstoichiometry in Y-Ba and Tl cuprates, which always appears as a critical parameter for superconductivity but whose influence on the microstructure differs in the different families. Cationic and anionic nonstoichiometry is connected with superconducting properties in order to understand their influence.

Commonalities in the various features of superconducting cuprates are discussed along with some recent findings on the relation between the electronic structure and the superconductivity of cuprates. The importance of Cu-O charge-transfer energy and the polarizability of CuO₂ sheets is pointed out.

Structure of high-temperature superconductor can be regarded as alternating stack of the metallic CuO₂ sheet and insulating ionic layer (block layer). Various physico-chemical properties of each compound are mostly governed by the constituent block layers and their combination. Using the concept of the block layer, we can design new layered cuprate compounds as prospective high-T_c superconductors and predict their structural details and carrier-doping characteristics.

A brief account of the structure and physical properties of the 90K superconductor, Y-123 is presented. The normal state properties and superconducting parameters are described in more detail. Effect of oxygen stoichiometry and chemical substitution at the various lattice sites on the structure and T_c behavior are also discussed.

Solid state chemistry of the superconducting copper oxides containing bismuth, thallium and lead is reviewed. Crystal chemistry and structure-property relationships are discussed in detail. The role of bismuth, thallium and lead in stabilizing the appropriate structures and in doping charge carriers (holes) in the CuO₂ sheets is pointed out. While the excess oxygen incorporated in the Bi₂O₂ layers dopes holes in the CuO₂ sheets of bismuth cuprates, the mechanism of doping in thallium cuprates is quite complex. Both a thallium vacancy in the TlO sheets and an internal redox mechanism involving overlap of empty Tl:6s band with the conduction band of CuO₂ sheets are likely to contribute to hole doping in thallium cuprates.

The structure of the Bi-based high T_c oxides having general formula Bi₂Sr₂Ca_n−1Cu_nO_y is complicated by the presence of an incommensurate structural modulation whose origin and influence on T_c have been controversial. We review our experimental work demonstrating that chemical doping studies are a powerful tool for unambiguously answering these questions. Cu can be completely replaced by a 3d-metal (Fe, Co and Mn) in the high T_c Bi based cuprates, giving new non-superconducting phases of general formula Bi₂Sr₂Ca_n−1M_nO_y, where n = 1, 2 and 3, and M is the 3d transition metal. These materials have a structural modulation that is commensurate. The structure determination shows that the modulation is accompanied by one extra row of oxygen atoms inserted in the Bi-O layers for each superstructure period a. Under well defined ambients (N₂) Bi can be replaced by Pb in the 3d-metal doped Bi phases to produce new compounds of formula BiPbSr₂MO₆ with no extra oxygens per formula unit. These phases do not have a structure modulation, consistent with our model of the modulation's origin and the absence of extra oxygen. In these unmodulated phases, the modulation can be induced (reversibly) if the material is heated under oxygen at 500°C. This shows for the first time the direct correlation between oxygen content and the structural modulation. To complete the modulation story, we synthesized new Pb-substituted Bi cuprates, Bi_2−xPb_xSr_2−yLa_yCuO₆ with x = y + 0.2, which superconduct at 28K but have no modulation. This indicates unambiguously that the structural modulation is neither a necessary nor sufficient condition for superconductivity among the Bi-based compounds.

The T'-phase electron-doped high T_c cuprate superconductors and their physical properties are reviewed. Topics discussed include the materials, crystal structure, superconductivity (pressure dependence of the superconducting critical temperature, temperature dependence of the anisotropic critical fields and related superconducting parameters, irreversibility line, fluxoid dynamics, etc.), magnetic ordering of Cu²⁺ and trivalent lanthanide ions, interplay between superconductivity and magnetism, and transport, thermal, magnetic, and spectroscopic studies of the normal state.

Effects of high pressure and high oxygen pressure on the formation, structure, oxygen content, and electrical and magnetic properties of Cu-oxides crystallizing in perovskite-related structures have been studied. Under 6GPa at 1223K ACuO₂ (A: Ba_1/3Sr_2/3 ~ Sr_1/3Ca_2/3) is stabilized in the Ca_0.84Sr_0.16CuO₂ type structure and R₂CuO₄ (R: Y, Dy, Ho, Er, Tm) in the Nd₂CuO₄-type structure. Very recently superconductivity with T_c=50K~100K has been detected in the A-Cu-O system, free from any rare earth ion, Bi³⁺, and Tl³⁺, treated at 6GPa and 1373K. La_2−xB_xCuO_4+z with B=Nd and Bi have been found to show large superconducting volume fractions of ≥10% after being treated under high oxygen pressures, 6GPa for B=Nd but only 50MPa for B=Bi. These fractions are several times as large as that of La₂CuO_4+z treated at an oxygen pressure of 6GPa. The known microscopic phase separation into nearly stoichiometric La₂CuO₄ and superconducting La₂CuO_4+z seems to be suppressed by the chemical substitution in the rock salt layers.

The chemical and structural aspects of the copper-less oxide superconducting systems are reviewed. Instability in one form or the other seems to be the key to finding newer superconducting systems. All the systems have at least one chemically tunable parameter, like concentration of substituent or oxygen content which can optimise the superconducting transition temperature. A comparison with copper oxide superconductors is made to bring about the commonalties and differences, which may provide lead in to understanding the superconducting state.

An overview of literature covering the preparation and characterization of La₂NiO_4+δ is presented in view of the close relationship to the layered cuprates. Particular emphasis is placed on the sensitive dependence of structure and physical properties on die concentration of excess lattice oxygen, δ. The La₂NiO_4+δ phase diagram exhibits structural, antiferromagnetic and metal/insulator phase transitions and at contains at least one phase separation region, similar to the La₂CuO_4+δ system. Whereas the structure and magnetic properties of the stoichiometric composition are reasonably well established, significant discrepancies exist with regard to the evolution of behavior with increasing oxygen content.

The following sections are included:

• Introduction

• Experimental procedure

• Main results of investigation
  • Binary and pseudobinary systems
    • Phase equilibria in Ba-Cu-O system and oxygen nonstoichiometry of individual phases
    • Thermodynamic properties of individual phases in the Ba-Cu-O system
    • Thermodynamic properties and phase relations in Y²O³-BaO system
  • Pseudoternary YO_1.5-BaO-CuO system
    • Phase relationships in system

• Thermodynamic properties of phases in Y-Ba-Cu-O system and phase stability
  • Literature review
  • Original studies

• Some applications
  • Thermodynamic analysis of atmospheric degradation of Y-Ba-Cu-O materials
  • The interaction of SC-materials with substrates and crucibles

• ACKNOWLEDGEMENTS

• REFERENCES

We review the important results obtained by investigating the electronic structure of the cuprate superconductors using high-energy spectroscopic techniques, with special emphasis on photoemission, soft x-ray absorption and electron energy loss spectroscopy. We discuss the probable description of the ground-state of these compounds in terms of the extent of mixed-valency and the symmetry of the doped hole-states.

Low-field microwave absorption (LFMA) is a highly sensitive, non-invasive technique to detect and characterize the superconducting phases in high temperature superconducting oxides. The LFMA signal depends on pinned magnetic flux, the relative contributions of intergranular and intragranular currents, the distribution and coupling of Josephson junctions and the variation in the magnetic field penetration depth. It is also sensitive to the applied field modulation, microwave power, and temperature. These interesting features of the microwave absorption are described and explained.

Highly textured Bi₂Sr₂CaCu₂O_x(2212) tapes were prepared by applying partial melting and slow cooling process for BiSrCaCuO/Ag composite prepared by doctor blade process. Microstructural studies indicated that initially an oriented structure of 2201 forms by growth from the melt and it subsequently trnasforms to the 2212 phase. The textured structure seems to initiate at the oxide surface and possibly from the oxide/Ag interface. Ag from the substrate also plays an important role in the grain alignment, by lowering the formation temperature of 2212 during cooling. J_c at 77K was greater than 10⁴A/cm²in zero magnetic field, however, it is very sensitive to an applied magnetic field and shows a large anisotropy with respect to field direction. These sensitivity and anisotropy are significantly reduced at lower temperatures. J_c's at 4.2K reach practical level(~l0⁵A/cm²) even in high magnetic fields above 20T and shows a small anisotropy factor, which should not limit practical application.

The physical vapour deposition techniques such as evaporation, sputtering and ion beam deposition used for preparing YBaCuO superconducting films are reviewed and their relative merits and demerits discussed. The characteristics of the films and their dependence on the deposition technique, parameters and post deposition annealing are also discussed. Laser deposition technique is not included as it appears in another article. Those techniques which are more suitable for preparing as deposited superconducting films are highlighted.

Deposition of high temperature superconductor thin films by the technique of pulsed excimer laser ablation is discussed giving several examples of studies related to process parameter optimization and development of barrier layer materials for realization of high quality films on different substrate materials. Processing approaches for incorporating device related features are also briefly described.

Magnetic properties and field modulated microwave absorption have been studied on different types of YBa₂Cu₃O₇ thin films, either textured or epitaxial. Hysteresis loops and remanent magnetization have been measured as a function of the maximum applied field in order to get insight into the flux penetration mechanisms. Pinning force and the irreversible line have been investigated on epitaxial samples, for different thicknesses. The magnetically modulated microwave response has been studied in the X-band. Harmonics of the frequency of the modulation field are found to depend on the history and on the granularity of the samples. The role of flux penetration at planar defects or grain boundaries is discussed in relation both with magnetic and field modulated microwave properties.